Abstract
Hydrogels are hydrophilic 3D networks that are able to ingest large amounts of water or biological fluids, and are potential candidates for biosensors, drug delivery vectors, energy harvester devices, and carriers or matrices for cells in tissue engineering. Natural polymers, e.g., cellulose, chitosan and starch, have excellent properties that afford fabrication of advanced hydrogel materials for biomedical applications: biodegradability, biocompatibility, non-toxicity, hydrophilicity, thermal and chemical stability, and the high capacity for swelling induced by facile synthetic modification, among other physicochemical properties. Hydrogels require variable time to reach an equilibrium swelling due to the variable diffusion rates of water sorption, capillary action, and other modalities. In this study, the nature, transport kinetics, and the role of water in the formation and structural stability of various types of hydrogels comprised of natural polymers are reviewed. Since water is an integral part of hydrogels that constitute a substantive portion of its composition, there is a need to obtain an improved understanding of the role of hydration in the structure, degree of swelling and the mechanical stability of such biomaterial hydrogels. The capacity of the polymer chains to swell in an aqueous solvent can be expressed by the rubber elasticity theory and other thermodynamic contributions; whereas the rate of water diffusion can be driven either by concentration gradient or chemical potential. An overview of fabrication strategies for various types of hydrogels is presented as well as their responsiveness to external stimuli, along with their potential utility in diverse and novel applications. This review aims to shed light on the role of hydration to the structure and function of hydrogels. In turn, this review will further contribute to the development of advanced materials, such as “injectable hydrogels” and super-adsorbents for applications in the field of environmental science and biomedicine.
Highlights
This review outlines knowledge gaps related to the role of water and hydration kinetics in various processes for swellable polymer networks
The swelling properties of polymer networks involve the ingression of large amounts water into the polymer matrix via diffusion, adsorption, and capillary action, where the elasticity of the crosslink points of the 3D network afford the swelling and contraction of the hydrogel constructs as a result of physical or chemical triggers
The role of water and the hydration kinetics of hydrogel systems were accounted for using the Flory-Huggins and the Fickian models
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The design strategy of super-porous and super-sorbent hydrogel materials for application in the environment and the biomedical fields demands the formation of highly functional 3D structures with favorable physicochemical and mechanical properties. Advanced hydrogel materials with a desirable water sorption/retention capacity and mechanical properties can be achieved via a composite formation strategy that employs natural and synthetic polymers with pre-determined physicochemical properties such as biodegradability, solubility, crystallinity, surface/textural properties, biological activities, and sensitivity to external stimuli. The use of natural polymers (e.g., alginate, collagen, starch, cellulose, and chitosan) in the fabrication of hydrogel materials has advanced the field of functional hydrogel systems This is related to the role of such biopolymer scaffolds to afford the formation of interpenetrating polymer networks (IPNs) with improved water diffusion/sorption and rheological properties [25,26,27]. Of advanced hydrogel materials with improved stability and mechanical strength, such as “injectable hydrogels” and super-sorbents, for applications in the environment and the biomedical fields
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